1. Field of the Invention
The present invention relates to a multiple differential demodulator used in a wireless communication system such as a WPAN and sensor network. More particularly, the invention relates to a multiple differential demodulator which does not employ square operations conducted by a correlator of a noncoherent detector in a conventional multiple differential noncoherent demodulator but applies a weighting value to the greater value of either a real part or an imaginary part to decrease phase offsets, thereby eliminating square loss.
2. Description of the Related Art
The term “ubiquitous” has been proposed recently to refer to a communication environment in which one can connect to a network at any time, any place. In addition, there have been active researches on small-scale wireless communication systems such as a Wireless Local Area Network (WLAN), Wireless Personal Area Network (WPAN), sensor network, Radio Frequency Identification (RFID), and the like rather than large-scale communication networks such as a cellular network.
Among these communication systems in particular, WPAN and sensor network require ultra-small size, low power consumption, and low price in addition to communication performance. Therefore, high-performance and expensive components used in the existing cellular communication systems or WLAN systems are hardly employed in the small-scale wireless communication systems including the WPAN and sensor network. However, if low-price components are adopted to lower the costs, frequency or phase offsets can occur, which requires a solution.
Therefore, there have been researches on a demodulator that performs well in a communication system with large frequency or phase offsets.
FIG. 1 is a block diagram illustrating a conventional multiple differential noncoherent demodulator proposed to compensate for frequency or phase offsets.
Referring to FIG. 1, the conventional multiple differential noncoherent demodulator 20 includes a multiple differentiator 21 for multi-delaying an I/Q signal outputted from an IF end 10 of a receiver to differentiate the signal, a plurality of noncoherent detectors 22 (only one is illustrated) for computing the correlations between the differentiated reception signals outputted from the multiple differentiator 21 and PN codes corresponding respectively to 16 symbols, and a maximum value selector 23 for comparing output results of the plurality of noncoherent detectors 22 to detect the maximum correlation value and determining a symbol of the PN code having the maximum correlation value as the symbol of the reception signal.
The conventional multiple differential noncoherent demodulator 20 can be described in greater detail as follows. An RF reception signal is frequency-converted into an IF signal at an RF end (not shown) of a receiver, separated and converted into a baseband I/Q signal by a mixer of the IF end 10. Then, the signal is sampled by an analogue/digital converter A/D and inputted into a demodulator 20 as a digital signal. The reception signal r(k) (k represents a sequence of a received packet) is a complex signal, where a real part is represented by “real{r(k)}” and an imaginary part by “imag{r(k)}.”
Thereafter, demodulation is executed on the digitized baseband signal, real{r(k)} and imag{r(k)}. First, the multiple differentiator 21 in the demodulator 20 complex-multiplies the digital signal (the signal received before the predetermined delay times) delayed by predetermined delay times 1Tc, 2Tc and 3Tc by a current reception signal to acquire differentiated values. FIG. 2 shows blocks of the multiple differentiator 21 in detail. The multiple differentiator 21 delays the signal r(k) received at the IF end 10 (FIG. 1) by delayers 211 to 213 having a plurality of predetermined delay times, e.g. Tc, 2Tc and3Tc, and conjugates the delayed signals by conjugators 214 to 216. Then, each of the multipliers 217 to 219 multiplies the reception signal r(k) by the delayed conjugated signals to output differentiated signals. Thus, a plurality of differentiated signals Dr,Tc(k), Dr,2Tc(k) and Dr,3Tc(k) delayed by different delay times are outputted from the multiple differentiator 21. Given that the signal r(k) received at the multiple differentiator 21 is represented as in Equation 1, the output signal of the multiple differentiator 21 can be represented as in Equation 2.r(k)=s(k)ej2πΔfk   Equation 1
                                                                                          D                  r                                ⁡                                  (                  k                  )                                            =                            ⁢                                                r                  ⁡                                      (                    k                    )                                                  ⁢                                                      r                    ⁡                                          (                                              k                        -                        N                                            )                                                        *                                                                                                        =                            ⁢                                                s                  ⁡                                      (                    k                    )                                                  ⁢                                  ⅇ                                      j2πΔ                    ⁢                                                                                  ⁢                    fk                                                  ⁢                                                      s                    ⁡                                          (                                              k                        -                        N                                            )                                                        *                                ⁢                                  ⅇ                                                            -                      j2πΔ                                        ⁢                                                                                  ⁢                                          f                      ⁡                                              (                                                  k                          -                          N                                                )                                                                                                                                                                    =                            ⁢                                                s                  ⁡                                      (                    k                    )                                                  ⁢                                                      s                    ⁡                                          (                                              k                        -                        N                                            )                                                        *                                ⁢                                  ⅇ                                      j2πΔ                    ⁢                                                                                  ⁢                    fN                                                                                                                          =                            ⁢                                                s                  ⁡                                      (                    k                    )                                                  ⁢                                                      s                    ⁡                                          (                                              k                        -                        N                                            )                                                        *                                ⁢                                  (                                                            cos                      ⁢                                                                                          ⁢                      2                      ⁢                      πΔ                      ⁢                                                                                          ⁢                      fN                                        +                                          j                      ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                      2                      ⁢                      πΔ                      ⁢                                                                                          ⁢                      fN                                                        )                                                                                        Equation        ⁢                                  ⁢        2            
In above Equations 1 and 2, s(k) is a transmission signal, i.e., a PN code of a symbol corresponding to the reception signal. Examining Equation 1, the reception signal r(k) includes channel distortion of ej2πΔfk in addition to the transmission signal s(k) (the PN code of the symbol), and the demodulator functions to restore the signal s(k) from the reception signal r(k). Here, Δfk is a value representing frequency offsets and ΔfN is a value representing phase offsets obtained from differentiating Δfk.
A plurality of noncoherent detectors 22 (FIG. 1) correlate the signals outputted from the multiple differentiator 21 with differentiated PN code signals of symbols to output correlation results. This process is described with reference to FIGS. 3 and 4.
FIG. 3 is a detailed block diagram of a noncoherent detector. As shown in FIG. 3, the noncoherent detector 22 (FIG. 1) includes first to third correlators 221 to 223 and a summer 224. The first to third correlators 221 to 223 complex-conjugate the differentiated signals by the delay times Tc, 2Tc and 3Tc with PN code signals differentiated (by each delay time) in the same fashion as the differentiated signals. The summer 224 sums output values from the first to third correlators 221 to 223 to output a correlation value Ei with the corresponding symbol, where i is a constant at least 0 and up to 15, referring to the corresponding symbol.
The first to third correlators 221 to 223 complex-multiply each of the plurality of differentiated signals Dr,Tc(k), Dr,2Tc(k) and Dr,3Tc(k) outputted from the multiple differentiator 21 (FIG. 1) by PN code differentiated signals Ds,Tc(k), Ds,2Tc(k) and Ds,3Tc(k) (also referred to as differentiated PN codes). The PN code differentiated signals are differentiated in the same fashion as the differentiated signals. One of the correlators is shown in greater detail in FIG. 4. FIG. 4 is a detailed block diagram of the first correlator 221, which has an identical structure with the second correlator 222 and the third correlator 223. As shown in FIG. 4, the correlator 221 includes first to fourth multipliers 241 to 244, an adder 245, a subtractor 246, two integrators 247 and 248, two square operators 249 and 250, and another adder 251. Each of the first to fourth multipliers 241 to 244 distinguishes the differentiated signal Dr,Tc(k) and the differentiated signal of the PN code Ds,Tc(k) into a real part and an imaginary part, and conducts multiplication between the real parts/imaginary parts or between the real and imaginary parts. The adder 245 adds output values of the first and second multipliers 241 and 242, and the subtractor 246 calculates difference between output values of the third and fourth multipliers 243 and 244. The two integrators 247 and 248 respectively integrate the output values of the adder 245 and the subtractor 246 for one symbol period. The two square operators 249 and 250 respectively square output values from the two integrators 247 and 248. Another adder 251 adds output values of the two square operators 249 and 250 to output a correlation result ETc.
When the differentiated signals and the differentiated PN code signals are complex-multiplied, four multiplication results are obtained from the first to fourth multipliers 241 to 244. Afterwards, the multiplication results are integrated by the integrators 247 and 248. Then, the real part and the imaginary part are squared respectively. The final results are added together to obtain the correlation result ETc.
The correlation results ETc, E2Tc and E3Tc for each delay time outputted from the first to third correlators 221 to 223 having the structure and operation described above (FIG. 3) are summed by the summer 224 (FIG. 3) to acquire a correlation value Ei between the reception signal and the symbol. The correlation value Ei outputted from the noncoherent detector 22 (FIG. 1) can be expressed as in Equation 3 below.
                    Ei        =                              ∑                          j              =              0                        3                    ⁢                                                                                    ∑                                      K                    =                    N                                    K                                ⁢                                  [                                                                                    D                        r                                            ⁡                                              (                        k                        )                                                              ⁢                                                                                            D                          s                                                ⁡                                                  (                          k                          )                                                                    *                                                        ]                                                                    2                                              Equation        ⁢                                  ⁢        3            
In above Equation 3, N represents a delay time and K represents the number of samples in a symbol period.
Then, the maximum value selector 23 (FIG. 1) compares the correlation values outputted from the plurality of noncoherent detectors 22 (FIG. 1) operated as described above to detect the maximum value, and determines the symbol value corresponding to the PN code with the maximum value as the demodulation value of the reception signal r(k).
According to the conventional multiple differential noncoherent demodulator having the configuration as described above, as the reception signal r(k) is converted to a differentiated reception signal (differentiated value of the reception signal) Dr(k), phase offsets remain as seen in Equation 2. Thus, the correlators of the noncoherent detector execute square operations to sum the squared values in order to compensate for the phase offsets. However, due to these square operations, a signal including noise is squared to result in square loss, thereby degrading transmission performance.